This paper deals with the origin of overpressure in the Paleogene strata, in south of Liaozhong depression, Bohai Bay Basin, through comprehensive application and analysis of various geological data and research methods, we suggest that hydrocarbon generation is the main mechanism. In some areas (W-D structure), there is overpressure formed by superposition of disequilibrium and hydrocarbon generation. The main evidences are as follows: (1) Except for W-D structure, logging curves generally do not have typical characteristics of high porosity and low density caused by disequilibrium; (2) The top depth of overpressure formed by disequilibrium in sedimentary background is inconsistent with the reality; (3) The overpressure top develops in the internal large mudstone section, with greatly varying depth range (from 2 200-3 100 m), and it is basically the same as hydrocarbon generation top of source rocks; (4) The relationship between formation effective stress with velocity and density shows that the overpressure points all fall on the “unloading” curve, and the overpressure characteristics are hydrocarbon generation; (5) The overpressure source rocks are still in large hydrocarbon generation stage, continue to provide guarantee for overpressure formation; (6) Overpressure formed by superimposition of disequilibrium and hydrocarbon generation is characterized by typical formation high porosity and low density, the relationship curves between effective stress and formation velocity and density formations show obvious unloading phenomena.